The use of polymerization catalysts having reduced particle size is increasingly requested in certain polymerization technologies, particularly in slurry polymerization. In fact, when transporting a solid particle in a liquid flow, for example in vertical upward direction, the efficiency of the transport depends on the properties of the liquid and of the solid particle. Considering a spherical particle being transported in an upward flowing liquid stream, the difference in velocity between the liquid and the solid particle (the ‘slip-velocity’) in this case is equal to the so-called ‘terminal velocity’ of that particle. The terminal velocity Vt is defined by the following formula:
      V    t    =                              4          ⁢                                          ⁢          gd                          3          ⁢                                          ⁢                      C            d                              ⁢              (                                            ρ              s                        -            ρ                    ρ                )            where                d=diameter of the spherical object        g=gravitational acceleration,        ρ=density of the fluid,        ρs=density of the object,        A=πd2/4=projected area of the sphere,        Cd=drag coefficient.        
It is clear that for a given liquid and particle density, the terminal velocity is reduced with reducing particle diameter. This means that lowering a particle's size reduces the velocity difference between that particle and the transporting liquid, making the transport more efficient.
In turn, the polymer particle size is function of the original catalyst particle size in view of the replica phenomenon. Thus, this is the reason why it is desirable to have catalyst components with reduced particle size.
One way to prepare this kind of catalyst comprises using catalyst precursors already having small particle size.
MgCl2. alcohol adducts and their use as catalyst precursors of catalyst components for the polymerization of olefins is well known in the art.
In WO98/44009 are disclosed MgCl2. alcohol adducts having improved characteristics and characterized by a particular X-ray diffraction spectrum, in which, in the range of 2θ diffraction angles between 5° and 15°, the three main diffraction lines are present at diffraction angles 2θ of 8.8±0.2°, 9.4±0.2° and 9.8±0.2°, the most intense diffraction lines being the one at 2θ=8.8±0.2°, the intensity of the other two diffraction lines being at least 0.2 times the intensity of the most intense diffraction line. Said adducts can be of formula MgCl2.mEtOH.nH2O where m is between 2.2 and 3.8 and n is between 0.01 and 0.6. These adducts are obtained by specific preparation methods involving the reaction between MgCl2 and alcohol under specific conditions such as long reaction times, absence of inert diluents or use of vaporized alcohol.
The molten adduct is then mixed with an oil immiscible with it, in order to create an emulsion which is subsequently quenched with cold liquid hydrocarbons thereby obtaining the solid spherical particles.
The particle size of the adduct (precursor) is function of the energy provided to the emulsion system and, maintaining constant all the other features (shape of the tank and stirrer, type of oil) is inversely related to the intensity of stirring. Thus, in order to produce precursor with reduced particle size, higher amount of energy, in particular higher stirring, should be provided.
In the alternative, as described in WO05/039745, can be used specific devices for preparing emulsions having the particles of the dispersed phase in suitable small size.
Both cases involve a complication of the process, and thus it would be advisable to find an easier way to have available catalyst precursors with reduced particle size.
It has also been proposed to use surfactants at a certain stage of the procedure with the aim to direct the production of the spherical adduct particles having specific features in terms of particle size and distribution. In U.S. Pat. No. 7,135,531 a surfactant is used in a reaction involving magnesium dichloride, alcohol, an ether and an alkyl silicate, in order to produce a large size catalyst precursor. Polymer surfactants like polyacrylates, polymethacrylates and polyalkylmethacrilates are preferred. Particularly preferred are the surfactants sold under the tradename Viscoplex® and especially those having the designation 1-254 and 1-256. Comparison of examples 1 and 2 indeed indicates that increasing the amount of Viscoplex 1-254 causes an increase of precursor particle size.